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Note that the example aggregate array_accum is comparable to the now

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built-in array_agg.  Per suggestion from Robert Haas.
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Tom Lane 2008-11-20 21:10:44 +00:00
parent 170b66a0c5
commit 3d7ac0d0b1
1 changed files with 13 additions and 10 deletions
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doc/src/sgml/xaggr.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/xaggr.sgml,v 1.35 2007/02/01 00:28:18 momjian Exp $ --> <!-- $PostgreSQL: pgsql/doc/src/sgml/xaggr.sgml,v 1.36 2008/11/20 21:10:44 tgl Exp $ -->
<sect1 id="xaggr"> <sect1 id="xaggr">
<title>User-Defined Aggregates</title> <title>User-Defined Aggregates</title>
@ -9,7 +9,7 @@
</indexterm> </indexterm>
<para> <para>
Aggregate functions in <productname>PostgreSQL</productname> Aggregate functions in <productname>PostgreSQL</productname>
are expressed in terms of <firstterm>state values</firstterm> are expressed in terms of <firstterm>state values</firstterm>
and <firstterm>state transition functions</firstterm>. and <firstterm>state transition functions</firstterm>.
That is, an aggregate operates using a state value that is updated That is, an aggregate operates using a state value that is updated
@ -41,7 +41,7 @@
aggregate to work on a data type for complex numbers, aggregate to work on a data type for complex numbers,
we only need the addition function for that data type. we only need the addition function for that data type.
The aggregate definition would be: The aggregate definition would be:
<screen> <screen>
CREATE AGGREGATE sum (complex) CREATE AGGREGATE sum (complex)
( (
@ -80,7 +80,7 @@ SELECT sum(a) FROM test_complex;
the transition function is marked <quote>strict</> (i.e., not to be called the transition function is marked <quote>strict</> (i.e., not to be called
for null inputs). for null inputs).
</para> </para>
<para> <para>
Another bit of default behavior for a <quote>strict</> transition function Another bit of default behavior for a <quote>strict</> transition function
is that the previous state value is retained unchanged whenever a is that the previous state value is retained unchanged whenever a
@ -89,7 +89,7 @@ SELECT sum(a) FROM test_complex;
transition function as strict; instead code it to test for null inputs and transition function as strict; instead code it to test for null inputs and
do whatever is needed. do whatever is needed.
</para> </para>
<para> <para>
<function>avg</> (average) is a more complex example of an aggregate. <function>avg</> (average) is a more complex example of an aggregate.
It requires It requires
@ -132,7 +132,10 @@ CREATE AGGREGATE array_accum (anyelement)
</programlisting> </programlisting>
Here, the actual state type for any aggregate call is the array type Here, the actual state type for any aggregate call is the array type
having the actual input type as elements. having the actual input type as elements. The behavior of the aggregate
is to concatenate all the inputs into an array of that type.
(Note: the built-in aggregate <function>array_agg</> provides similar
functionality, with better performance than this definition would have.)
</para> </para>
<para> <para>
@ -149,14 +152,14 @@ SELECT attrelid::regclass, array_accum(attname)
pg_tablespace | {spcname,spcowner,spclocation,spcacl} pg_tablespace | {spcname,spcowner,spclocation,spcacl}
(1 row) (1 row)
SELECT attrelid::regclass, array_accum(atttypid) SELECT attrelid::regclass, array_accum(atttypid::regtype)
FROM pg_attribute FROM pg_attribute
WHERE attnum &gt; 0 AND attrelid = 'pg_tablespace'::regclass WHERE attnum &gt; 0 AND attrelid = 'pg_tablespace'::regclass
GROUP BY attrelid; GROUP BY attrelid;
attrelid | array_accum attrelid | array_accum
---------------+----------------- ---------------+---------------------------
pg_tablespace | {19,26,25,1034} pg_tablespace | {name,oid,text,aclitem[]}
(1 row) (1 row)
</programlisting> </programlisting>
</para> </para>